Operation of LEDs in ophthalmic instruments

Abstract

An ophthalmic instrument is equipped with LEDs (6). In addition to the LEDs (6), the instrument has a voltage source (B), an inductor (L), and a capacitor (C). The method for operating the instrument comprises at least one charging sequence (Z1) and one illumination sequence (Z2). The charging sequence includes at least a first and at least a second phase (P1, P2). In the first phase (P1), the voltage source (B) and the inductor (L) are connected in series. In the second phase (P2), the inductor (L) and the capacitor (C) are connected in series. The illumination sequence (Z2) comprises at least a third and at least a fourth phase. In the third phase (P3), the LED(s) (6), the inductor (L), and the capacitor (C) are connected in series. In the fourth phase (P4), the LED(s) and the inductor (L) are connected in series. In the charging sequence (Z1), the inductor (L) serves to transfer energy from the voltage source (B) to the capacitor (C).In the lighting sequence, the coil (L) is used for current regulation or control of the LEDs (6).

Description

Area

[0001] The invention relates to a method for operating at least one LED of an ophthalmic tool and to a correspondingly designed ophthalmic tool, i.e. a tool that is used in the training or in real applications of ophthalmology. background

[0002] Ophthalmic instruments, such as fundus lenses, surgical instruments, or manipulators, are used in eye examinations, treatments, and / or training. Modern ophthalmic microscopes or training devices can be designed to optically detect such instruments using cameras and record their position, for example, to mark them on a screen, provide user feedback, or log actions.

[0003] To simplify tool recognition, it is known to equip the tools with LEDs that emit light signals, especially pulsed light signals that can be detected by a camera.

[0004] In other applications, one or more LEDs on such a tool can be used, for example, as indicator elements that emit light pulses to indicate a specific operating state.

[0005] To power the LEDs, the tool can be equipped with a power cable, but this is often considered cumbersome in practice. Alternatively, the tool could be equipped with an integrated power source, such as a battery or accumulator. However, in this case, electronics are required to adapt the voltage of the power source to that of the LEDs and supply the LEDs with the desired current. Depiction

[0006] The task therefore is to create a method of the type mentioned above as well as an ophthalmological tool that provides a space-saving, efficient power supply for one or more LEDs.

[0007] This task is solved by the method or tool according to the independent claims.

[0008] The method is therefore used to operate at least one LED in an ophthalmic instrument, the instrument comprising, in addition to the LED, at least a voltage source, an inductor, and a capacitor. In many versions, the instrument will have multiple LEDs.

[0009] The method comprises at least one charging sequence and one lighting sequence, wherein the LED(s) is / are excited to light up over at least part of the lighting sequence.

[0010] The loading sequence has at least one first and at least one second phase, and in many implementations it has multiple first and second phases that are executed alternately. The first and second phases are as follows: - In the first phase, the voltage source and the coil are connected in series in a first circuit, so that a current flows through the coil and a magnetic field is generated in the coil. In this first phase, the capacitor is not in series with the voltage source and the coil, i.e., it is not in the first circuit. - In the second phase, the voltage source is disconnected from the coil, i.e., it is removed from the circuit. The coil and the capacitor are connected in series in a second circuit, so that the magnetic field is at least partially reduced via an induced current through the capacitor, thereby charging the capacitor.

[0011] The lighting sequence has at least one third and at least one fourth phase, and in many versions it has several third and fourth phases which are executed alternately. The third and fourth phases are as follows: In the third phase, the LED(s), the inductor, and the capacitor are connected in series in a third circuit, so that an increasing current flows from the capacitor through the LED(s) until the current exceeds a first current threshold. Here, the capacitor is at least partially discharged via the inductor and the LED(s), with the inductor, depending on its inductance, ensuring that the current does not rise abruptly to excessively high values. The third phase ends when the current exceeds the aforementioned first current threshold, which can be achieved, for example, through appropriate timing or current measurement. In the fourth phase, the capacitor is disconnected from the inductor, so it is no longer in the circuit. The LED(s) and the inductor are connected in series in a fourth circuit, causing a decreasing current to flow through the LED until it falls below a second current threshold. Here, the magnetic field of the inductor is at least partially dissipated by an induced current, and the current decreases at a rate dependent on the inductor's inductance. The fourth phase ends when the current falls below the second threshold, which can be achieved, for example, through appropriate timing or current measurement.

[0012] In this process, the coil thus has two functions. During the charging sequence, it performs the voltage conversion to charge the capacitor to a voltage adapted to the LED(s). During the illumination sequence, the current from the capacitor to the LED(s) is also routed through the coil, so that the coil limits the rate of rise and fall of the current and thus forms part of an energy-efficient current limiting mechanism, which is achieved through the sequence of the third and fourth phases.

[0013] Since a coil occupies a fairly large volume, this double coil design can save space.

[0014] As mentioned, many versions of the tool incorporate multiple LEDs, which simplifies, for example, tool position detection. These LEDs are operated in series during the lighting sequence, i.e., in the third and fourth phases, so that they have defined relative brightness levels, e.g., all being approximately the same brightness. Since series-connected LEDs require a relatively high supply voltage, charging them via the coil is advantageous because it allows the voltage to be increased from the typically much lower nominal voltage of the power source.

[0015] The charging sequence can have several first and second phases, which are executed alternately. This allows for a higher voltage to be built up across the capacitor and / or a reduction in the current flow from the voltage source. However, it is also conceivable to use only one first and one second phase, provided that sufficient charge can be transferred to the capacitor.

[0016] The lighting sequence can have multiple third and fourth phases, which are executed alternately. This allows the difference between the first and second current thresholds to be reduced while still achieving longer light pulses. However, it is also conceivable to use only one third and one fourth phase, provided this generates a sufficiently long light pulse.

[0017] An ophthalmological instrument, in particular a fundus lens, with which the procedure can be implemented, comprises an LED, a voltage source, a coil, and a capacitor. It also includes a control unit designed to carry out the procedure.

[0018] In the second phase, the voltage source can be located in the second circuit, i.e., it can be connected in series with the inductor and the capacitor. In this case, the induced voltage of the inductor and the voltage of the voltage source are added together.

[0019] The tool can have at least one first, one second, one third, and one fourth switch. These can be opened and closed by the control unit to form the first, second, third, and fourth circuits.

[0020] An "open" switch is an electrically insulating switch, and a "closed" switch is an electrically conductive switch.

[0021] Although it is conceivable to provide more than four switches, it turns out that four such switches are sufficient to selectively form all four circuits.

[0022] In other embodiments, some of the switches can be replaced by switching diodes, specifically when a voltage of the first polarity is present across a given switch in all phases when it is closed and a voltage of the second, reverse polarity is present when it is open. Thus, the tool can also have several, but no more than three, switches and at least one switching diode, where the switches can be opened and closed by the control unit to selectively form the first, second, third, and fourth circuits together with the at least one switching diode.

[0023] However, a solution using one or more switching diodes is less efficient than a solution using only switches due to the voltage drop across the switching diodes.

[0024] A switching diode is understood to be a diode without light emission; that is, an LED is not considered a switching diode in this context. In other words, the switching diode(s) are provided in addition to the LEDs in these specifications.

[0025] The switches can be designed as transistors, for example. Brief description of the drawings

[0026] Further features, advantages, and applications will emerge from the dependent requirements and from the following description using the figures. These will show: Fig. 1. An example of an ophthalmic tool, Fig. 2 a first example of a switching arrangement, Fig. 3 the state of the switches of the arrangement of Fig. 2 in five different phases of operation and Fig. 4 the course of the coil current and the capacitor voltage over one cycle. Examples of tool design

[0027] Fig. Figure 1 shows a possible example of a tool in the form of a fundus lens. During operation, this lens is manually inserted by the user between the ophthalmic microscope and the patient's eye by the ophthalmologist. This allows the imaging optics to be changed so that the microscope displays the structures of the retina instead of the anterior chamber. In training settings, such a lens can also be used as a training device, which is held in front of the eye of an artificial patient.

[0028] The tool has a frame 2 that holds a lens 4. Several LEDs 6 are arranged within the frame (eight LEDs are shown in this case), which emit light pulses in the visible or NIR range. These pulses can be detected with spatial resolution, for example, by a camera on the microscope or by a camera on the artificial patient. In this way, it is possible, for example, to automatically detect the presence and / or position of the fundus lens. This can be used, for example, to adjust an operating mode of the microscope (e.g., to change the illumination) or to provide the user with feedback on the handling of the tool in a training or work situation.

[0029] In the case of a training tool, lens 2 can also be omitted, and the LEDs can be distributed across the cross-section of the entire tool.

[0030] However, as mentioned, the present technique can also be used for other tools, such as ophthalmic surgical instruments (like knives or needles), manipulators (like tweezers or hooks), syringes or other objects used in eye examinations or eye treatments and / or ophthalmic training.

[0031] Furthermore, as already mentioned, at least one LED can also be used, for example, for display purposes or for lighting.

[0032] The device may have a single LED 6 if the LED is to be used, for example, as a signal light, or if the position of the tool, but not its orientation, is to be detected, for example, if it is a simple pointer. However, in many versions, the tool has several LEDs 6, so that, for example, its orientation can also be detected, and / or so that it can be detected from different directions. Switching arrangement

[0033] Fig. Figure 2 shows an example of a switching arrangement for operating the LEDs 6. Such a switching arrangement can be integrated into the tool.

[0034] The circuit arrangement has a voltage source B. This is advantageously a rechargeable or non-rechargeable battery, e.g. a button cell, since the present technology makes it possible to generate strong light pulses even with voltage sources of low voltage and low current output.

[0035] Since space is limited in most tools, compact power sources are advantageous, especially those that don't require multiple electrochemical cells connected in series. However, such power sources typically have a relatively low nominal voltage (Ub), usually below 4 V. For example, a lithium-ion button cell has a voltage of 3.7 V.

[0036] The circuit also includes a capacitor C. This is used to supply current to the LED(s). As described below, it is charged to a capacitor voltage Uc sufficient to operate the 6 LEDs. If several LEDs are connected in series, the capacitor voltage must be sufficiently high, as a typical voltage drop of around 2 V occurs across each LED during operation.

[0037] The circuit arrangement also includes a converter 8. This converter has two functions. Firstly, it converts the voltage Ub of the voltage source B to the desired capacitor voltage Uc, which in most cases requires increasing the voltage. Secondly, it also serves as a current controller or current limiter for operating the LEDs. These functions should be implemented in an energy-efficient manner and within a small footprint.

[0038] Furthermore, the switching arrangement has a control unit 10. This can be operated, for example, by the voltage source B and generates control signals for controlling the switches of the switching arrangement, as described later.

[0039] In the embodiment shown, the switching arrangement has a first switch S1, a second switch S2, a third switch S3 and a fourth switch S4, which are controlled by the controller 10.

[0040] Furthermore, the switching arrangement has a coil L.

[0041] The 6 LEDs can be connected in series, as mentioned earlier. In this case, only current limiting is required, and the light emission is approximately the same for all LEDs. In the circuit diagram, the LEDs are labeled D1 to Dn, where n is advantageously greater than 2, e.g., 6 or 8.

[0042] Specifically, the first switch S1 connects a first pole 12 of the voltage source B to a first side 14 of the coil L and to a first side 16 of the LEDs 6.

[0043] The second switch S2 connects a second side 18 of the LEDs 6 to a second pole 20 of the voltage source B and to a first terminal 22 of the capacitor C.

[0044] The third switch S3 connects a second side 24 of the coil L to the second pole 20 of the voltage source B and the first terminal 22 of the capacitor C.

[0045] The fourth switch S4 connects the second side 24 of the coil to a second terminal 26 of the capacitor C. Operation

[0046] The following describes a possible operation of the switching arrangement based on: Fig. 3 and Fig. 4 described.

[0047] The switching arrangement is operated in cycles. Each cycle comprises a charging sequence Z1 and a lighting sequence Z2 (see figure). Fig. 4) In the charging sequence Z1, the capacitor C is charged from the voltage source B, and in the lighting sequence Z2, the LEDs are operated.

[0048] In the charging sequence Z1, the switching arrangement is operated in at least one first operating phase P1 and at least one second operating phase P2. Advantageously, each charging sequence comprises several first and second operating phases in alternating order.

[0049] In the lighting sequence Z2, the switching arrangement is operated in at least one third operating phase P3 and at least one fourth operating phase P4. Advantageously, each lighting sequence includes several third and fourth operating phases in alternating sequence.

[0050] In addition, the switching arrangement can also be put into a standby operating phase P0.

[0051] The operating phases will be referred to as "phases" in the following.

[0052] Fig. Figure 4 shows the sequence of a cycle with charging sequence Z1 and lighting sequence Z2.

[0053] Before the cycle begins, the switching arrangement can be in phase P0. In this phase P0 (see below) Fig. 3) All switches S1 ... S4 are open. No current flows. The signal LEDs 6 are off. The circuit is in standby mode. Phase P0 is therefore referred to as the standby phase in the following. Alternatively, the beginning of a cycle can also follow directly after the end of the previous cycle, without an intervening phase P0.

[0054] At the beginning of a cycle, no current flows through the coil L. The voltage Uc across the capacitor is at a starting value U0. This starting value depends on whether a cycle occurred shortly beforehand or whether the switching arrangement has not been used for a longer period. Accordingly, U0 can be > 0 or = 0. In the present example, according to Fig. 4. It is assumed that several cycles are performed in quick succession, so that U0 is greater than 0, usually even greater than the voltage Ub.

[0055] The cycle begins with a phase P1 (see switch position according to Fig. 3) In this phase, the first and third switches S1, S3 are closed, and the second and fourth switches S2, S4 are open. Thus, the voltage source B and the coil L are in a first circuit formed by the series arrangement of the voltage source B, the first switch S1, the coil L, and the third switch S3. A current IL therefore builds up through the coil L, the rate of rise of which uA is defined by the inductance of the coil (see the graph above). Fig. 4).

[0056] In the second phase P2, the first and fourth switches S1, S4 are closed and the second and third switches S2, S3 are open (see below). Fig. 3) Thus, the coil L, the capacitor C, and the voltage source B are in a second circuit, formed by the series arrangement of the voltage source B, the first switch S1, the coil L, the fourth switch S4, and the capacitor. During the transition from the first phase P1 to the second phase P2, the inductance of coil 3 initially keeps the current through the coil constant, and an induced voltage is generated, which is added to the voltage Ub and applied across the capacitor C. The current from the coil charges the capacitor C, and its voltage increases, while simultaneously the current decreases again (cf. Fig. 4).

[0057] The second phase P2 is followed by a first phase P1, in which the current through the coil L is increased again, while the capacitor C is decoupled and its voltage Uc remains constant ( Fig. 4). Then another phase P2 follows, in which the capacitor C is charged and Uc increases.

[0058] This process is repeated until a desired final voltage U1 is reached across capacitor C and the desired charge is stored in capacitor C. The final voltage U1 is higher than the minimum operating voltage for the series arrangement of LEDs. For example, the minimum operating voltage per LED is slightly more than 2 V, meaning approximately 16 V for a series arrangement of eight LEDs. U1 should be higher than this minimum operating voltage so that sufficient charge is available to maintain the current throughout the entire light pulse. If U0 corresponds to the minimum operating voltage of the series arrangement of LEDs, then U1 should, roughly estimated, be greater than U0 + I. LED ·t Puls / C, where I LED the average current through the illuminated LEDs and t Puls Specify the duration of the light pulse.

[0059] In many applications, the final voltage U1 is greater than the nominal voltage Ub of the voltage source B and is, for example, at least 10 V.

[0060] The lighting sequence Z2 now begins, whereby an optional standby phase P0 may be inserted between the charging sequence Z1 and the lighting sequence Z2, in which, for example, a trigger signal is awaited in order to send out the light pulse.

[0061] The lighting sequence Z2 begins with a third phase P3. In the third phase, the second switch S2 and the fourth switch S4 are closed, and the first switch S1 and the third switch S3 are open. This places the LEDs 6, the inductor L, and the capacitor C in a third circuit, formed by the series connection of the inductor L, the fourth switch S4, the capacitor C, the second switch S2, and the LEDs 6. Consequently, a current IL builds up through the inductor L, the rate of rise of which is defined by the inductor's inductance (see the graph above). Fig. 4) This current flows in the opposite direction to the current during the charging sequence. It increases in strength until it exceeds a first current threshold I1. Then the next phase P4 begins.

[0062] In the fourth phase P4, the second and third switches S2, S3 are closed, and the first and fourth switches S1, S4 are open. This places the LED and the inductor L in a fourth circuit, formed by the series connection of inductor L, the third switch S3, the second switch S2, and the LEDs 6. Capacitor C is not part of this fourth circuit. Therefore, the current IL now decreases again until it falls below a second current threshold I2. Then the next phase P3 begins.

[0063] As from Fig. As can be seen in Figure 4, the current through the coil, and thus through the LEDs, fluctuates between the values ​​I1 and I2, which are dimensioned so that they are slightly above and below the desired luminous current of the LEDs 6.

[0064] When the lighting sequence Z2 is finished, it is advantageous for the switching arrangement to remain in phase P4 for a slightly longer period, so that the current through the coil L is reduced as completely as possible.

[0065] The switching arrangement can then, for example, transition to the standby phase P0.

[0066] In this way, a pulsed operation of the LEDs 6 results, in that the LEDs 6 are dark during the charging sequence Z1 and a first part of the lighting sequence Z2 and only light up during a second part of the lighting sequence Z2.

[0067] The cycle of charging sequence Z1 and lighting sequence Z2 can be repeated periodically, for example. Circuit variants

[0068] Fig. Figure 2 shows only one possible configuration of the switching arrangement.

[0069] For example, although this makes the control somewhat more difficult, the first switch S1 can also be located in the line to the second pole 20 of the voltage source B (position 30 in Fig. 2), or the fourth switch can be located in the line to the first terminal 22 of capacitor C (position 32 in Fig. 2).

[0070] In another embodiment, the first switch S1 can be replaced by a switching diode whose anode is connected to the positive terminal 12 of the voltage source B, at least for applications in which the nominal voltage Ub of the voltage source is below the operating voltage of the LEDs arranged in series (see switching diode 34 shown in dashed lines in Fig. 2).

[0071] In the execution according Fig. In the second phase P2, the voltage of the voltage source B and the induced voltage of the coil L with the same sign are added together to produce a higher total voltage of the same polarity.

[0072] The circuit arrangement according to Fig. The device has a first switch S1, which is directly connected to one pole of the voltage source B and thus interrupts the current flow through the voltage source when open. For example, the first switch can be located between the voltage source B and the coil L. The first switch S1 is closed in the first phase P1 so that a current can be generated in the coil L. The first switch is open at least in the third and fourth phases so that the voltage source B is disconnected during the lighting sequence.

[0073] In addition, the tool can have a second switch S2 in this case. The first switch S1 connects a first terminal 12 of the voltage source to a first side 14 of the coil L and to a first side 16 of the LED, and the second switch S2 connects a second side 18 of the LED to a second terminal 20 of the voltage source. The second switch S2 is open in the first and second phases and closed at least in the fourth phase, thus ensuring that no unwanted current flows through the LEDs 6 during the charging sequence.

[0074] In terms of the type of variant according to Fig. 2 is the first switch S1 in the second phase P2 closed, and the second switch S2 is closed in the third phase P3 and in the fourth phase P4.

[0075] Furthermore, the circuit arrangement has according to Fig. 2. A fourth switch S4 is directly connected to one terminal of capacitor C and thus, when open, interrupts the current flow through the capacitor. For example, the fourth switch can be located between the inductor L and the capacitor C. The fourth switch S4 is closed in the second and third phases P2, P3 and open in the first and fourth phases P1, P4, i.e., it disconnects the capacitor C from the inductor in those phases where the voltage Uc across the capacitor C should not change. Remarks

[0076] Controller 10 could, for example, be a simple pulse generator, perhaps with a counter that is incremented by a clock. The counter addresses a memory chip in which four-bit words are stored. Each bit controls the state of a switch. In this way, the sequences Z1 and Z2 can be traversed by incrementing the counter.

[0077] However, the control unit 10 can also be implemented as a programmable module.

[0078] Specifically, the durations of the different phases P1 - P4 can be estimated from the voltage Ub of the voltage source B, the inductance of the coil L, the capacitance of the capacitor C, as well as the target current and the operating voltage of the LEDs 6, so that a simple pulse generator can control the entire cycle without a control loop.

[0079] More complex control systems can, for example, also monitor at least one or more currents or voltages in the switching arrangement in order to achieve more flexible control.

[0080] For example, the voltage of capacitor C can be measured during charging, e.g. via a voltage divider in parallel to capacitor C, the tap of which is fed to a threshold detector of the control unit 10.

[0081] Additionally or alternatively, the current through the LEDs can be measured during operation, e.g. by measuring the voltage across a shunt resistor at the cathode of the lowest LED (towards the negative terminal of the voltage source).

[0082] Thus, the respective end of at least one of the phases, preferably all phases, can be determined via a voltage and / or current measurement.

[0083] In other versions, however, the respective end of at least one of the phases, preferably all phases, can also be determined purely by time control, i.e., the end of the phases is not determined by measuring a current or voltage in one of the circuits.

[0084] The charging sequence can, for example, have a total duration of 5 to 10 ms, while the illumination sequence has a duration of less than 100 µs, e.g., 12 to 15 µs. However, other durations are possible depending on the component dimensions and requirements.

[0085] Switches S1-S4 can be implemented as switching transistors, particularly MOSFET transistors. Switches S1, S2, and S3 can, for example, be implemented as n-MOS transistors, while switch S4 can be implemented as a p-MOS transistor. To control switch S4, whose drain and source can be temporarily at relatively high voltages, a pull-up resistor can be provided to raise the gate voltage to the source voltage. The gate voltage can then be pulled down by another transistor.

[0086] Other types of transistor control may include, for example, additional auxiliary transistors, pull-up or pull-down resistors, or optocouplers to generate the necessary gate voltages if required.

[0087] The in Fig.However, the circuit variant shown in 2 has the advantage that, in particular, the gate voltages of transistors for switches S1 - S3 are in a range that can be easily generated by the controller 10, even if the controller 10 is operated by the voltage source.

[0088] Capacitor C can be implemented as a single capacitor (as shown in the figures). However, it can also be implemented, for example, as several capacitors connected in parallel. This allows the current through the individual capacitors to be reduced, especially during the illumination phase.

[0089] While preferred embodiments are described in the present application, it should be clearly pointed out that the invention is not limited to these and can also be implemented in other ways within the scope of the following claims.

Claims

[1] Method for operating at least one LED (6) of an ophthalmic instrument, wherein the instrument, in addition to the LED (6), comprises a voltage source (B), an inductor (L) and a capacitor (C), wherein the method comprises a charging sequence (Z1) and a lighting sequence (Z2), and wherein the LED is excited to light over at least a part of the lighting sequence (Z2), and wherein the charging sequence (Z1) comprises at least a first and at least a second phase (P1, P2), wherein - in the first phase (P1) the voltage source (B) and the coil (L) are connected in series in a first circuit, but not the capacitor (C), so that a current is built up through the coil (L) and a magnetic field is built up in the coil (L), and - in the second phase (P2) the coil (L) and the capacitor (C) are connected in series in a second circuit, so that the magnetic field is at least partially reduced via an induced current through the capacitor (C) and thereby the capacitor (C) is charged, and wherein the lighting sequence (Z2) has at least a third and at least a fourth phase (P3, P4), wherein - in the third phase (P3) the LED (6), the coil (L) and the capacitor (C) are connected in series in a third circuit, so that an increasing current flows through the LED (6) until the current exceeds a first current threshold (I1), and - in the fourth phase (P4) the capacitor (C) is disconnected from the coil (L), and the LED (6) and the coil (L) are connected in series in a fourth circuit, so that a decreasing current flows through the LED (6) until the current falls below a second current threshold (I2). [2] Method according to one of the preceding claims, wherein several LEDs (6) are provided which are connected in series in the lighting sequence (Z2). [3] Method according to any of the preceding claims, wherein the loading sequence (Z1) comprises several first and several second phases (P1, P2) which are executed alternately. [4] Method according to one of the preceding claims, wherein the lighting sequence (Z2) comprises several third and several fourth phases (P3, P4) which are executed alternately. [5] Method according to one of the preceding claims, wherein in the second phase (P2) the voltage source (B) is located in the second circuit. [6] Method according to any of the preceding claims, wherein the tool has a first switch (S1) which is directly connected to a pole of the voltage source (B) and thus interrupts a current flow through the voltage source when open, wherein the first switch (S1) is closed in the first phase (P1) and is open at least in the third and fourth phases (P3, P4). [7] Method according to claim 6, wherein the tool has a second switch (S2), wherein the first switch (S1) connects a first pole (12) of the voltage source (B) to a first side (14) of the coil (L) and to a first side (16) of the LED (6), and the second switch (S2) connects a second side (18) of the LED (6) to a second pole (20) of the voltage source (B), wherein the second switch (S2) is open in the first and second phase (P2) and closed in the fourth phase (P4). [8] Method according to claim 7, wherein the first switch (S1) is closed in the second phase (P2) and the second switch (S2) is closed in the third phase (P3). [9] Method according to any of the preceding claims, wherein the tool has a fourth switch (S4) which is directly connected to a terminal of the capacitor (C) and thus interrupts a current flow through the capacitor (C) when open, wherein the fourth switch (S4) is closed in the second and third phases (P2, P3) and open in the first and fourth phases (P1, P4). [10] Method according to one of the preceding claims, wherein an end of at least one of the phases (P1 - P4), preferably of all phases, is determined by a voltage and / or current measurement. [11] Method according to any of the preceding claims, wherein an end of at least one of the phases (P1 - P4), preferably of all phases, is time-controlled. [12] Method according to any of the preceding claims, wherein the voltage source (B) is a battery. [13] Ophthalmological tool, in particular fundus lens, comprising an LED (6), a voltage source (B), a coil (L) and a capacitor (C), and a control (10), wherein the control (10) is configured to carry out the method according to one of the preceding claims. [14] Tool according to claim 13, wherein the tool has a first, a second, a third and a fourth switch (S1 - S4) which can be opened and closed by the control (10) to form the first, the second, the third and the fourth circuit. [15] Tool according to claim 14, wherein the tool has exactly four switches (S1 - S4). [16] Tool according to one of claims 14 or 15, wherein the fourth switch (S4) is arranged between the coil (L) and the capacitor (C) and is closed to form the second circuit and the third circuit. [17] Tool according to claim 16, wherein the tool has several, but not more than three, switches (S1 - S4) and at least one switching diode (34), wherein the switches (S1 - S4) can be opened and closed by the control (10) to form, together with the at least one switching diode (34), the first, second, third and fourth circuits. [18] Tool according to one of claims 13 to 17 with several LEDs connected in series (6).

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